The present invention relates generally to semiconductor chip wire bonding devices, and similar bonding apparatus, and particularly to a method and apparatus for locating a ball bond formed on a pad of a semiconductor chip, in order to perform automated optical inspection of wire bonding in such a device.
Semiconductor devices, such as integrated circuit chips, are electrically connected to leads on a lead frame by a process known as wire bonding. The wire bonding operation involves bonding a wire to electrically connect pads residing on a die (semiconductor chip) to a lead in a lead frame. Once the chip and lead frame have been wire bonded, they can be packaged in ceramic or plastic to form an integrated circuit device. A post-process inspection step, commonly called the third optical inspection, typically involves locating the position of all bonds on the device, the wire connections and the wire heights using optical means.
Heretofore the third optical inspection has been accomplished only after the device is completely bonded and sent to a separate machine or operator. In the majority of cases, the inspection is done by a human operator using a microscope. This manual method can be time-consuming and costly.
Separate machines are available to perform this step, but this requires another piece of capital equipment in the production line. Additionally, a post-process inspection machine has a more difficult time locating the bond to perform a successful inspection because all the information about the chip that was available during the bonding operation, such as exact pad and frame positions and information about other detail have been lost.
This is further complicated by the fact that most semiconductor chips have a considerable amount of visual detail (such as the images of the circuits themselves) which must be circumvented in analyzing the post-bond image to find the bonds. In post-process inspections, some of this detail can be mistaken for parts of the bonds.
Automatically locating the center of a ball bond in an image is required to accurately detect the presence or absence of the ball bond on a pad on a semiconductor die; to find the bond's precise location on a pad, and to serve as a principal step in automating the inspection of the quality of a connection to a pad.
Machine vision systems or image processing systems (systems that capture images, digitize them and use a computer to perform image analysis) have been used on wirebonding machines to align devices and guide the machine for correct bonding placement, but have heretofore not been used during the process to locate the bonds formed and inspect them.
Where post process inspections are automated, the visual detail that is unrelated to the bond may be misinterpreted as part of the bond in a post-process inspection, giving rise to erroneous acceptance or rejection rates. Visual imperfections on the pads and leads caused by probe marks, discoloration, or imperfect illumination further complicate these difficulties. These blemishes may be misconstrued as defects in the bonding process, without the information that was available during the bonding operation.
An additional problem encountered in attempting to perform the inspection in-process can be created by the differences caused by bonding itself. Depending on the type of bonding process and equipment used, heating, cooling, movement and other mechanical factors can create alignment problems for images taken before and after the bonding process, thus making it harder to locate the bond. Thus, ball bonds are typically located by hand in a manual inspection procedure, since there are no accepted techniques in the field for automatically locating ball bonds in images.
Using normalized correlation templates is an accepted machine vision technique to find objects in imagery, but with this technique the template is extremely specific to the object at hand and not flexible enough to handle variations in size or shape. Ball bonds, although generally circular or elliptical, are likely to vary significantly in size and shape.
One approach which has been tried involves foregoing any attempt to automatically locate the ball bond after bonding, because of the difficulties mentioned above. In this approach a system would assume that the bonds have been (correctly) placed by the wire bonder machine, as guided by the machine vision system, and the system would use those nominal locations on the semiconductor die as the precise ball bond location. The problem with this approach is that a typical wire bonder machine's inaccuracies in positioning will very frequently cause the later inspection step to fail.